Thermal insulation and method of producing the same

ABSTRACT

Provided is a thermal insulation having both excellent thermal insulating performance and excellent strength, and a method of producing the same. A method of producing a thermal insulation according to the present invention includes curing (S 2 ) a dry-pressed compact, including silica fine particles each having an average particle diameter of 50 nm or less and a reinforcement fiber, at a relative humidity of 70% or more.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationJP 2009-239326, filed on Oct. 16, 2009, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal insulation and a method ofproducing the same, and more particularly, to an improvement in thestrength of a thermal insulation.

2. Description of the Related Art

Conventionally, as thermal insulations having low thermal conductivityand being excellent in thermal insulating performance, there have beenknown ones which are obtained by mixing silica fine particles as amaterial having a low thermal conductivity, an inorganic fiber, and abinder, and then subjecting the mixture to press forming, followed bymachining (for example, JP 11-513349 A and JP 11-514959 A).

However, the above-mentioned conventional technologies have involved aproblem in that use of the binder requires, for example, removal of oil,which causes reduction in the strength of the thermal insulations. Inaddition, the use of the binder increases environmental impact. Asdescribed above, there has been a problem in that the removal of oil orthe like involves increases in the number of steps and required time andenergy when the binder is used.

In contrast, it is also possible to enhance the strength of a thermalinsulation by increasing the density of the thermal insulation throughthe adjustment of pressing pressure without use of a binder. However,this case has involved a problem, for example, in that the thermalinsulating performance of the thermal insulation lowers because theincrease in the density of the thermal insulation involves an increasein the solid heat transfer.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedproblems. An object of the present invention is to provide a thermalinsulation having both excellent thermal insulating performance andexcellent strength, and a method of producing the same.

A method of producing a thermal insulation according to one embodimentof the present invention for solving the above-mentioned problemsincludes curing a dry-pressed compact including silica fine particleseach having an average particle diameter of 50 nm or less and areinforcement fiber at a relative humidity of 70% or more. According tothe present invention, there can be provided a method of producing athermal insulation having both excellent thermal insulating performanceand excellent strength.

Further, the dry-pressed compact may be free of a binder. Further, thedry-pressed compact may include 50 to 98 mass of the silica fineparticles and 2 to 20 mass % of the reinforcement fiber.

Further, the dry-pressed compact may include at least one of analkaline-earth metal hydroxide and an alkali metal hydroxide. In thiscase, the dry-pressed compact may include at least one of thealkaline-earth metal hydroxide and the alkali metal hydroxide each at0.1 to 10 parts by weight with respect to 100 parts by weight of a rawmaterial for the thermal insulation including the silica fine particlesand the reinforcement fiber.

A thermal insulation according to one embodiment of the presentinvention for solving the above-mentioned problems is produced by anyone of the methods described above. According to the present invention,there can be provided a thermal insulation having both excellent thermalinsulating performance and excellent strength.

A thermal insulation according to one embodiment of the presentinvention for solving the above-mentioned problems includes silica fineparticles each having an average particle diameter of 50 nm or less anda reinforcement fiber, in which the thermal insulation has a bulkdensity of 190 to 600 kg/m³ and has a compressive strength of 0.65 MPaor more. According to the present invention, there can be provided athermal insulation having both excellent thermal insulating performanceand excellent strength.

Further, the thermal insulation may be free of a binder. Further, thethermal insulation may include 50 to 98 mass % of the silica fineparticles and 2 to 20 mass % of the reinforcement fiber.

Further, the thermal insulation may include at least one of analkaline-earth metal and an alkali metal in addition to the silica fineparticles and the reinforcement fiber. In this case, the thermalinsulation may include at least one of the alkaline-earth metal and thealkali metal each at 0.1 to 10 parts by weight with respect to 100 partsby weight of a raw material for a thermal insulation including thesilica fine particles and the reinforcement fiber. Further, the thermalinsulation may have a thermal conductivity of 0.05 W/(m·K) or less at600° C.

According to the present invention, there can be provided the thermalinsulations each having both excellent thermal insulating performanceand excellent strength, and the methods of producing a thermalinsulation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an explanatory diagram illustrating main steps included in oneexample of a method of producing a thermal insulation according to oneembodiment of the present invention;

FIG. 2A is an explanatory diagram of a part of a mechanism by which thestrength of a thermal insulation is improved by curing in the method ofproducing a thermal insulation according to one embodiment of thepresent invention;

FIG. 2B is an explanatory diagram of another part of the mechanism bywhich the strength of the thermal insulation is improved by the curingin the method of producing a thermal insulation according to oneembodiment of the present invention;

FIG. 2C is an explanatory diagram of still another part of the mechanismby which the strength of the thermal insulation is improved by thecuring in the method of producing a thermal insulation according to oneembodiment of the present invention;

FIG. 3 shows one example of results obtained by investigating thecompressive strength of thermal insulations by changing curingconditions in an example according to one embodiment of the presentinvention;

FIG. 4 is an explanatory diagram showing examples of electron microscopephotographs of thermal insulations obtained in the example according toone embodiment of the present invention;

FIG. 5A is an explanatory diagram illustrating one example of X-raydiffraction results of a thermal insulation obtained in the exampleaccording to one embodiment of the present invention;

FIG. 5B is an explanatory diagram illustrating another example of theX-ray diffraction results of the thermal insulation obtained in theexample according to one embodiment of the present invention;

FIG. 5C is an explanatory diagram illustrating still another example ofthe X-ray diffraction results of the thermal insulation obtained in theexample according to one embodiment of the present invention;

FIG. 5D is an explanatory diagram illustrating still another example ofthe X-ray diffraction results of the thermal insulation obtained in theexample according to one embodiment of the present invention;

FIG. 6A is an explanatory diagram illustrating one example of acorrelation of peak values in an X-ray diffraction and a compressivestrength with respect to a thermal insulation obtained in the exampleaccording to one embodiment of the present invention;

FIG. 6B is an explanatory diagram illustrating another example of acorrelation of peak values in an X-ray diffraction and a compressivestrength with respect to a thermal insulation obtained in the exampleaccording to one embodiment of the present invention;

FIG. 6C is an explanatory diagram illustrating still another example ofa correlation of peak values in an X-ray diffraction and a compressivestrength with respect to a thermal insulation obtained in the exampleaccording to one embodiment of the present invention;

FIG. 7 shows one example of results obtained by investigating thecompressive strength of thermal insulations by changing an additionamount of calcium hydroxide in an example according to one embodiment ofthe present invention;

FIG. 8 shows one example of results obtained by investigating thecompressive strength of thermal insulations by changing curing time inan example according to one embodiment of the present invention;

FIG. 9 is an explanatory diagram illustrating one example of resultsobtained by investigating the compressive strength of a thermalinsulation by changing curing temperature in an example according to oneembodiment of the present invention;

FIG. 10A is an explanatory diagram showing another example of anelectron microscope photograph of a thermal insulation obtained in theexample according to one embodiment of the present invention;

FIG. 10B is an explanatory diagram showing still another example of anelectron microscope photograph of a thermal insulation obtained in theexample according to one embodiment of the present invention; and

FIG. 11 is an explanatory diagram illustrating one example of resultsobtained by investigating the compressive strength of thermalinsulations by changing the kind of an alkaline-earth metal hydroxide inan example according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, one embodiment of the present invention is described. Notethat the present invention is not limited to this embodiment.

First, a method of producing a thermal insulation according to thisembodiment (hereinafter referred to as “method of this embodiment”) isdescribed. The method of this embodiment is a method of producing athermal insulation involving curing a dry-pressed compact, includingsilica fine particles each having an average particle diameter of 50 nmor less and a reinforcement fiber, at a relative humidity of 70% ormore.

FIG. 1 is an explanatory diagram illustrating main steps included in oneexample of the method of this embodiment. In the example illustrated inFIG. 1, the method of this embodiment includes a preparation step S1 ofpreparing the dry-pressed compact, a curing step S2 of curing thedry-pressed compact at high humidity, and a drying step S3 of drying thedry-pressed compact after the curing.

In the preparation step S1, a raw material for a thermal insulationincluding silica fine particles and a reinforcement fiber is prepared.The silica fine particles are not particularly limited as long as eachthe silica fine particles have an average particle diameter of 50 nm orless. Any one kind of silica fine particles may be used alone or two ormore kinds of silica fine particles may be used in optional combination.

That is, it is possible to use, as the silica fine particles, forexample, dry silica fine particles (anhydrous silica fine particles)produced by a gas phase method or wet silica fine particles produced bya wet method. In particular, the dry silica fine particles can bepreferably used. To be specific, it is possible to preferably use, forexample, fumed silica fine particles produced by the gas phase method.In particular, hydrophilic fumed silica fine particles can be preferablyused.

The average particle diameter of the silica fine particles can be setmore specifically to, for example, 5 nm or more and 50 nm or less. Thesilica (SiO₂) content of the silica fine particles is preferably, forexample, 95 wt % or more. The thermal conductivity at 25° C. of thesilica fine particles is preferably, for example, 0.01 W/(m·K) or less.The specific surface area of the silica fine particles by a BET methodis preferably, for example, 50 m²/g or more. To be more specific, thespecific surface area can be set to, for example, 50 m²/g or more and400 m²/g or less, or can be set more preferably to 100 m²/g or more and300 m²/g or less.

The reinforcement fiber is not particularly limited as long as thereinforcement fiber can reinforce the thermal insulation. An inorganicfiber or an organic fiber may be used or both of them may be used.

The inorganic fiber is not particularly limited as long as the inorganicfiber can be used as the reinforcement fiber. Any one kind of inorganicfiber may be used alone, or two or more kinds of inorganic fibers may beused in optional combination. To be specific, it is possible to use atleast one kind of inorganic fiber selected from the group consisting of,for example, a silica-alumina fiber, a silica fiber, an alumina fiber, azirconia fiber, an alkaline-earth silicate fiber, a glass fiber, rockwool, and a basalt fiber. Note that the alkaline-earth silicate fiber isa bio-soluble inorganic fiber. That is, a non-bio-soluble inorganicfiber or a bio-soluble inorganic fiber may be used, or both of them maybe used, as the inorganic fiber.

The thermal conductivity of the inorganic fiber at 400° C. is, forexample, preferably 0.08 W/(m·K) or less or more preferably 0.04 W/(m·K)or less. It is possible to preferably use, for example, a silica-basedfiber such as a silica-alumina fiber or a silica fiber as the inorganicfiber with low thermal conductivity described above.

The fiber length of the inorganic fiber is, for example, preferably 1 mmor more and 10 mm or less, more preferably 1 mm or more and 7 mm orless, or particularly preferably 3 mm or more and 5 mm or less. When thefiber length is less than 1 mm, it may be impossible to suitably alignthe inorganic fiber. As a result, the mechanical strength of the thermalinsulation may be insufficient. When the fiber length is more than 10mm, the powder flow property of the raw material for a thermalinsulation at the time of forming may be impaired, a reduction in theformability may occur and density irregularity may cause reduction inthe workability.

The average fiber diameter of the inorganic fiber is preferably, forexample, 15 μm or less. To be more specific, the average fiber diameteris preferably, for example, 5 μm or more and 15 μm or less. When theaverage fiber diameter is more than 15 μm, the inorganic fiber maybecome liable to break in some cases. As a result, the strength of thethermal insulation may be insufficient. Thus, it is possible topreferably use an inorganic fiber having a fiber length of 1 mm or moreand 10 mm or less, and having an average fiber diameter of 15 μm orless, as the inorganic fiber.

The organic fiber is not particularly limited as long as the organicfiber can be used as the reinforcement fiber. Any one kind of organicfiber may be used alone, or two or more kinds of organic fibers may beused in optional combination. To be specific, it is possible to use atleast one kind of organic fiber selected from the group consisting of,for example, an aramid fiber, a polyethylene fiber, a polypropylenefiber, and a polyolefin fiber.

The fiber length of the organic fiber is, for example, preferably 1 mmor more and 10 mm or less, more preferably 2 mm or more and 7 mm orless, or particularly preferably 3 mm or more and 5 mm or less. When thefiber length is less than 1 mm, it may be impossible to suitably alignthe organic fibers. As a result, the mechanical strength of the thermalinsulation may be insufficient. When the fiber length is more than 10mm, the powder flow property of the raw material for a thermalinsulation at the time of forming may be impaired, a reduction in theformability may occur and density irregularity may cause reduction inthe workability.

The average fiber diameter of the organic fiber is preferably, forexample, 15 μm or less. To be more specific, the average fiber diameteris preferably, for example, 5 μm or more and 15 μm or less. When theaverage fiber diameter is more than 15 μm, the organic fiber may becomeliable to break in some cases. As a result, the strength of the thermalinsulation may be insufficient. Thus, it is possible to preferably usean organic fiber having a fiber length of 1 mm or more and 10 mm or lessand having an average fiber diameter of 15 μm or less as the organicfiber.

The dry-pressed compact can be produced by mixing such silica fineparticles and reinforcement fiber as described above in a dry manner toproduce a dry mixture and then subjecting the dry mixture to dry-pressforming.

To be specific, the dry-pressed compact is produced, for example, bysubjecting a raw material for a thermal insulation including a drypowder of the silica fine particles and a dry powder of thereinforcement fiber to dry mixing by using a predetermined mixingapparatus, and then filling a predetermined forming mold with theresultant dry mixture to carry out dry-press forming. Note that mixingand forming in a dry system facilitate the control of the raw materialand compact, and can effectively reduce a time required for production,compared with mixing and forming in a wet system.

The dry-pressed compact may include, for example, 50 to 98 mass % of thesilica fine particles and 2 to 20 mass % of the reinforcement fiber, or65 to 80 mass % of the silica fine particles and 5 to 18 mass % of thereinforcement fiber. When the content of the reinforcement fiber is lessthan 2 mass %, the strength of the thermal insulation may beinsufficient. When the content of the reinforcement fiber is more than20 mass %, the powder flow property of the raw material for a thermalinsulation at the time of forming may be impaired. As a result,reduction in the formability may occur and the density irregularity maycause reduction in the workability.

Further, when the dry-pressed compact includes only the silica fineparticles and the reinforcement fiber, the dry-pressed compact, forexample, may include 80 to 98 mass % of the silica fine particles and 2to 20 mass % of the reinforcement fiber so that the total contentthereof reaches 100 masse, may include preferably 82 to 98 mass % of thesilica fine particles and 2 to 18 mass % of the reinforcement fiber sothat the total content thereof reaches 100 mass %, or may include morepreferably 85 to 97 mass % of the silica fine particles and 3 to 15 mass% of the reinforcement fiber so that the total content thereof reaches100 mass %. When the content of the reinforcement fiber is less than 2mass %, the strength of the thermal insulation may be insufficient. Whenthe content of the reinforcement fiber is more than 20 mass %, thepowder flow property of the raw material for a thermal insulation at thetime of forming may be impaired. As a result, reduction in theformability may occur and density irregularity may cause reduction inthe workability.

Further, the dry-pressed compact may be free of a binder. That is, inthe method of this embodiment, the strength of the thermal insulationcan be effectively improved by a curing treatment described later, andhence it is not necessary to use a binder. In this case, the dry-pressedcompact is substantially free of a binder that has been conventionallyused, such as an inorganic binder, for example, a water glass adhesive,or an organic binder such as a resin. Thus, conventional problemsinvolved in the use of binders can be certainly avoided. Further, inthis case, dry-press forming is not particularly limited, and can becarried out, for example, at a temperature of 5° C. or more and 60° C.or less.

Further, the dry-pressed compact may also include at least one of analkaline-earth metal hydroxide and an alkali metal hydroxide. Thealkaline-earth metal hydroxide is not particularly limited as long asthe alkaline-earth metal hydroxide can be used as a strong base. Any onekind of alkaline-earth metal hydroxide may be used alone, or two or morekinds of alkaline-earth metal hydroxides may be used in optionalcombination. To be specific, it is possible to use at least one kind ofalkaline-earth metal hydroxide selected from the group consisting of,for example, calcium hydroxide, magnesium hydroxide, strontiumhydroxide, and barium hydroxide. In particular, calcium hydroxide can bepreferably used.

The alkali metal hydroxide is not particularly limited as long as thealkali metal hydroxide can be used as a strong base. Any one kind ofalkali metal hydroxide may be used alone, or two or more kinds of alkalimetal hydroxides may be used in optional combination. To be specific, itis possible to use at least one kind of alkali metal hydroxide selectedfrom the group consisting of, for example, sodium hydroxide, potassiumhydroxide, and lithium hydroxide.

The dry-pressed compact may include, for example, at least one of thealkaline-earth metal hydroxide and the alkali metal hydroxide each at0.1 to 10 parts by weight with respect to 100 parts by weight of the rawmaterial for a thermal insulation including the silica fine particlesand the reinforcement fiber. That is, in this case, the dry-pressedcompact may include one of the alkaline-earth metal hydroxide and thealkali metal hydroxide at 0.1 to 10 parts by weight, or may include thealkaline-earth metal hydroxide and the alkali metal hydroxide at 0.2 to20 parts by weight in total. The content of at least one of thealkaline-earth metal hydroxide and the alkali metal hydroxide may beset, for example, to 1 to 7 parts by weight or even to 2 to 5 parts byweight.

The dry-pressed compact can be produced by subjecting a dry powder ofthe silica fine particles, a dry powder of the reinforcement fiber, anda dry powder of at least one of the alkaline-earth metal hydroxide andthe alkali metal hydroxide to dry mixing, and then subjecting theresultant dry mixture to dry-press forming.

The dry-pressed compact may also further include other components. Thatis, the dry-pressed compact may also include, for example, a thermalradiation scattering material. The thermal radiation scattering materialis not particularly limited as long as the thermal radiation scatteringmaterial can reduce heat transfer caused by thermal radiation. Any onekind of thermal radiation scattering material may be used alone or twoor more kinds of thermal radiation scattering materials may be used inoptional combination.

To be specific, it is possible to use at least one kind of thermalradiation scattering material selected from the group consisting ofsilicon carbide, zirconia, and titania. Further, the thermal radiationscattering material preferably has an average particle diameter of, forexample, 50 μm or less, or to be more specific, 1 to 50 μm, and has aspecific refractive index of, for example, 1.25 or more with respect tothe light having a wavelength of 1 μm or more.

When the thermal radiation scattering material is used, the dry-pressedcompact, for example, may include preferably 50 to 93 mass % of thesilica fine particles, 2 to 20 mass % of the reinforcement fiber, and 5to 40 mass % of the thermal radiation scattering material, or mayinclude more preferably 65 to 80 mass % of the silica fine particles, 5to 18 mass % of the reinforcement fiber, and 15 to 30 mass % of thethermal radiation scattering material.

In the subsequent curing step S2, the dry-pressed compact prepared inthe preparation step S1 is cured under the relative humidity of 70% ormore. The relative humidity in the curing may be set, for example, to75% or more, to 80% or more, or even to 85% or more. Further, the curingmay also be carried out at a relative humidity of more than 85%.

The curing is carried out by keeping the dry-pressed compact for apredetermined time under such a high humidity environment as describedabove. To be specific, the dry-pressed compact can be subjected to highhumidity curing, for example, by placing the dry-pressed compact in athermo-hygrostat in which temperature and humidity are each set to apredetermined value, or in an autoclave in which arrival temperature isset to a predetermined value, and leaving the dry-pressed compact tostand for a predetermined time.

The temperature at which the curing is carried out may be arbitrarilyset within a range in which the effect of the curing can be provided. Tobe specific, the curing temperature, for example, may be set to 40° C.or more, is set to preferably 60° C. or more, more preferably 80° C. ormore, or particularly preferably 90° C. or more. By increasing thecuring temperature, it is possible to reduce the curing time until theeffect of the curing is provided. The upper limit of the curingtemperature is not particularly limited and can be set to, for example,95° C. or less. Note that when the dry-pressed compact includes analkaline-earth metal hydroxide, the curing temperature may preferably beset to 100° C. or less, or less than 100° C. in some cases. Further, thecuring temperature may be set to, for example, less than 40° C. as well.

Further, the curing may also be carried out under a pressurizedcondition. In this case, the curing temperature may be arbitrarily setwithin a range in which the effect of the curing can be provided. To bespecific, the curing temperature under a pressurized condition, forexample, may be set to 100 to 200° C. or may be even set to 120 to 170°C. By carrying out the curing under such a pressurized condition asdescribed above, it is expected to reduce the curing time until theeffect of the curing is provided.

The time for carrying out the curing may be arbitrarily set within arange in which the effect of the curing can be provided. To be specific,the curing time, for example, may be set to 2 hours or more, or ispreferably set to 6 hours or more. Increasing the curing time can leadto the enhancement of the effect of the curing.

To be more specific, when the dry-pressed compact is free of both thealkaline-earth metal hydroxide and the alkali metal hydroxide, a longercuring time is preferred. Further, when the dry-pressed compact includesa relatively small amount (for example, 0.1 to 2 parts by weight withrespect to 100 parts by weight of the raw material for a thermalinsulation including the silica fine particles and the reinforcementfiber) of at least one of the alkaline-earth metal hydroxide and thealkali metal hydroxide, the curing time is preferably set to 6 hours ormore and 100 hours or less. In the meantime, when the dry-pressedcompact includes a relatively large amount (for example, more than 2parts by weight and 20 parts by weight or less with respect to 100 partsby weight of the raw material for a thermal insulation including thesilica fine particles and the reinforcement fiber) of at least one ofthe alkaline-earth metal hydroxide and the alkali metal hydroxide, thecuring time is preferably set to 12 hours or less, or more preferably 6hours or less.

Note that curing conditions are not limited to the above-mentionedexamples, and may be arbitrarily set within a range in which the effectof the curing can be provided. That is, the curing conditions may besuitably adjusted, for example, so that the strength (for example,compressive strength) and thermal conductivity of the thermal insulationproduced by the method of this embodiment fall within such predeterminedranges as described later. Further, the curing time is not limited tothe above-mentioned examples, and may be suitably determined, forexample, depending on other curing conditions such as temperature andhumidity.

In the subsequent drying step S3, the dry-pressed compact cured in thecuring step S2 is dried. That is, water is removed in the drying stepS3, the water having penetrated into the dry-pressed compact at the timeof the curing and being derived from water vapor. The drying method isnot particularly limited as long as the drying method can removeunnecessary water from the dry-pressed compact. That is, for example, bykeeping the dry-pressed compact at a temperature of 100° C. or more, thedry-pressed compact can be dried efficiently.

In the method of this embodiment, the dry-pressed compact having gonethrough the curing and the drying is thus finally obtained as a thermalinsulation. According to the method of this embodiment, it is possibleto produce a thermal insulation having both excellent thermal insulatingperformance and excellent strength. That is, according to the method ofthis embodiment, it is possible to effectively improve the strength of athermal insulation without increasing the density of the thermalinsulation. Further, according to the method of this embodiment, it ispossible to produce a thermal insulation having sufficient strengthwithout using a binder.

FIGS. 2A to 2C are explanatory diagrams of a mechanism by which thestrength of a thermal insulation is improved by high humidity curing inthe method of this embodiment. Here, as illustrated in FIGS. 2A to 2C,attention is focused on two adjacent silica fine particles P1 and P2 outof the silica fine particles included in the dry-pressed compact, tothereby describe the mechanism. As the mechanism by which the strengthof a thermal insulation is improved by high humidity curing, thefollowing mechanism is considered.

That is, first, as illustrated in FIG. 2A, an extremely fine gap V (forexample, an ultrafine pore having a size of about several nm) is presentbetween the silica fine particles P1 and P2 included in the dry-pressedcompact before curing. Next, curing in which the dry-pressed compact iskept under a high humidity atmosphere is started. Then, as illustratedin FIG. 2B, capillary condensation of water vapor forms a bridgestructure B formed of a liquid containing condensed water as a maincomponent between the silica fine particles P1 and P2.

Further, the dry-pressed compact is successively kept under the highhumidity atmosphere. Then, as illustrated in FIG. 2C with arrows, silicais eluted from the silica fine particles P1 and P2, forming the bridgestructure B containing the eluted silica between the silica fineparticles P1 and P2. The elution reaction of silica must be thefollowing silicate reaction: “SiO₂+2H₂O→H₄SiO₄→H⁺+H₃SiO₄ ⁻.”

Next, the dry-pressed compact after the curing is dried. Then, thebridge structure B formed between the silica fine particles P1 and P2 ishardened. The formation of the bridge structure B described above canenhance the strength of the thermal insulation effectively. Note that asimilar bridge structure is also formed between a silica fine particleand a reinforcement fiber.

Further, when the dry-pressed compact includes at least one of analkaline-earth metal hydroxide and an alkali metal hydroxide, suchimprovement in the strength as described above can be promoted. As aresult, curing time can be effectively reduced. This is probably becausea highly basic environment suitable for the elution of silica from thesilica fine particles P1 and P2 is formed in the inside of thedry-pressed compact owing to the presence of the alkaline-earth metalhydroxide or the alkali metal hydroxide.

That is, it is considered that the use of a strong base such as thealkaline-earth metal hydroxide or the alkali metal hydroxide promotesthe elution of silica from the silica fine particles P1 and P2 in thecuring, resulting in the achievement of improvement in the strength ofthe thermal insulation in a short time. Note that this case includes theformation of a bridge structure B, including at least one of analkaline-earth metal and an alkali metal in addition to silica, betweenthe silica fine particles P1 and P2.

A thermal insulation according to this embodiment (hereinafter referredto as “thermal insulation of this embodiment”) can be preferablyproduced by the method of this embodiment described above. The thermalinsulation of this embodiment can have sufficient strength while havingrelatively low density. That is, the thermal insulation of thisembodiment can be, for example, a thermal insulation including silicafine particles each having an average particle diameter of 50 nm or lessand a reinforcement fiber, having a bulk density of 190 to 600 kg/m³,and having a compressive strength of 0.65 MPa or more.

The bulk density of the thermal insulation of this embodiment may bealso set to, for example, 190 to 450 kg/m³ or 190 to 300 kg/m³. Thecompressive strength of the thermal insulation of this embodiment may bealso set to, for example, 0.7 MPa or more or 0.75 MPa or more. Note thatthe compressive strength can be measured by using a predeterminedcompression testing apparatus such as a commercially available universaltesting apparatus (TENSILON RTC-1150A, Orientec Co., Ltd.). To bespecific, for example, a test piece obtained by cutting the thermalinsulation of this embodiment so that the test piece has a dimension of30 mm×30 mm×15 mm is loaded with a weight on the press surface (30 mm×30mm) in the direction perpendicular to the press surface, and the weightof the load (MPa) at the time of fracture of the test piece is definedas the compressive strength. When the thermal insulation of thisembodiment has a plate shape, the compressive strength can be evaluatedas compressive strength in the thickness direction of the thermalinsulation of this embodiment (that is, fracture strength at the timewhen a pair of surfaces extending in the longitudinal direction andhaving the largest area are compressed).

The thermal insulation of this embodiment, for example, may include 50to 98 mass % of the silica fine particles and 2 to 20 mass % of thereinforcement fiber, or may include 65 to 80 mass % of the silica fineparticles and 5 to 18 mass % of the reinforcement fiber. When thecontent of the reinforcement fiber is less than 2 mass %, the strengthof the thermal insulation of this embodiment may be insufficient. Whenthe content of the reinforcement fiber is more than 20 mass %, thepowder flow property of the raw material for the thermal insulation ofthis embodiment at the time of forming may be impaired. As a result,reduction in the formability may occur and density irregularity maycause reduction in the workability.

Further, when the thermal insulation of this embodiment includes onlythe silica fine particles and the reinforcement fiber, the thermalinsulation of this embodiment, for example, may include 80 to 98 mass %of the silica fine particles and 2 to 20 mass % of the reinforcementfiber so that the total content thereof reaches 100 mass %, may includepreferably 82 to 98 mass % of the silica fine particles and 2 to 18 mass% of the reinforcement fiber so that the total content thereof reaches100 mass %, or may include more preferably 85 to 97 mass % of the silicafine particles and 3 to 15 mass % of the reinforcement fiber so that thetotal content thereof reaches 100 mass %. When the content of thereinforcement fiber is less than 2 mass %, the strength of the thermalinsulation of this embodiment may be insufficient. When the content ofthe reinforcement fiber is more than 20 mass %, the powder flow propertyof the raw material for the thermal insulation of this embodiment at thetime of forming may be impaired. As a result, reduction in theformability may occur and density irregularity may cause reduction inthe workability.

Further, the thermal insulation of this embodiment may be free of abinder. That is, the thermal insulation of this embodiment can obtainsufficient strength by curing as described above, and hence it is notnecessary to use a binder. In this case, the thermal insulation of thisembodiment is substantially free of a binder that has beenconventionally used, such as an inorganic binder, for example, a waterglass adhesive or an organic binder, for example, a resin. Thus,conventional problems involved in the use of binders can be reliablyavoided.

Further, the thermal insulation of this embodiment may include at leastone of an alkaline-earth metal and an alkali metal in addition to thesilica fine particles and the reinforcement fiber. That is, the thermalinsulation of this embodiment may include a metal derived from at leastone of the alkaline-earth metal hydroxide and the alkali metal hydroxideused in the curing.

To be specific, the thermal insulation of this embodiment may include atleast one kind of alkaline-earth metal selected from the groupconsisting of, for example, calcium, magnesium, strontium, and barium.In particular, the thermal insulation of this embodiment preferablyincludes calcium. Further, the thermal insulation of this embodiment mayinclude at least one kind of alkali metal selected from the groupconsisting of, for example, sodium, potassium, and lithium.

The thermal insulation of this embodiment may include, for example, atleast one of the alkaline-earth metal and the alkali metal, each at 0.1to 10 parts by weight with respect to 100 parts by weight of a rawmaterial for the thermal insulation of this embodiment including thesilica fine particles and the reinforcement fiber. That is, in thiscase, the thermal insulation of this embodiment, for example, mayinclude one of the alkaline-earth metal and the alkali metal at 0.1 to10 parts by weight, or may include the alkaline-earth metal and thealkali metal at 0.2 to 20 parts by weight in total. Further, the contentof at least one of the alkaline-earth metal and the alkali metal may beset, for example, to 1 to 7 parts by weight or even to 2 to 5 parts byweight.

The thermal insulation of this embodiment may further include othercomponents. That is, the thermal insulation of this embodiment may alsoinclude, for example, a thermal radiation scattering material. Thethermal radiation scattering material is not particularly limited aslong as the thermal radiation scattering material can reduce heattransfer caused by thermal radiation. Any one kind of thermal radiationscattering material may be used alone, or two or more kinds of thermalradiation scattering materials may be used in optional combination.

To be specific, it is possible to use at least one kind of thermalradiation scattering material selected from the group consisting ofsilicon carbide, zirconia, and titania. Further, the thermal radiationscattering material preferably has an average particle diameter of, forexample, 50 μm or less, or to be more specific, 1 to 50 μm, and has aspecific refractive index of, for example, 1.25 or more with respect tothe light having a wavelength of 1 μm or more.

When the thermal radiation scattering material is used, the thermalinsulation of this embodiment, for example, may include 50 to 93 mass %of the silica fine particles, 2 to 20 mass % of the reinforcement fiber,and 5 to 40 mass % of the thermal radiation scattering material, or maymore preferably include 65 to 80 mass % of the silica fine particles, 5to 18 mass % of the reinforcement fiber, and 15 to 30 mass % of thethermal radiation scattering material.

Further, the thermal insulation of this embodiment can have excellentthermal insulating performance. That is, the thermal insulation of thisembodiment has obtained sufficient strength without increasing itsdensity, unlike conventional thermal insulations, and hence reduction inthermal insulating performance due to the increase of solid heattransfer can be effectively avoided. To be specific, the thermalinsulation of this embodiment can be a thermal insulation having athermal conductivity at 600° C. of 0.05 W/(m·K) or less. The thermalconductivity at 600° C. of the thermal insulation of this embodiment canalso be preferably set to 0.04 W/(m·K) or less.

Note that the thermal insulation of this embodiment has a structure inwhich each primary particles of the silica fine particles having anaverage particle diameter of 50 nm or less is assembled by anintermolecular force or the like to form secondary particles, which aredispersed among pieces of the reinforcement fiber. Further, becausesilica fine particles are used in the thermal insulation of thisembodiment, the thermal insulation of this embodiment internally has astructure of nanopores smaller than the mean free path of an airmolecule, and hence the thermal insulation of this embodiment can exertexcellent thermal insulating performance in a wide temperature range offrom a low temperature region to a high temperature region.

Further, the thermal insulation of this embodiment may be a thermalinsulation having a peculiar structure formed through high humiditycuring. That is, the thermal insulation of this embodiment can be, forexample, a thermal insulation including silica fine particles eachhaving an average particle diameter of 50 nm or less and a reinforcementfiber, and having a bridge structure containing silica between thesilica fine particles. The bridge structure is, as described above,formed by capillary condensation of water vapor and contains silicaeluted from the silica fine particles.

Further, the bridge structure may also include at least one of analkaline-earth metal and an alkali metal. That is, the bridge structurein this case includes, as described above, at least one of thealkaline-earth metal and the alkali metal derived from at least one ofthe alkaline-earth metal hydroxide and the alkali metal hydroxide usedin the curing.

Further, the thermal insulation of this embodiment may include calciumsilicate. That is, for example, when the thermal insulation of thisembodiment is produced through high humidity curing in which calciumhydroxide is added, calcium silicate may be generated inside the thermalinsulation of this embodiment because of a chemical reaction between asilica component eluted from the silica fine particles and the calciumhydroxide. Thus, the thermal insulation of this embodiment may includecalcium silicate generated through the high humidity curing in thebridge structure formed between silica fine particles or in anotherpart.

As described above, the thermal insulation of this embodiment can haveboth excellent thermal insulating performance and high strength whilehaving relatively low density. Thus, the thermal insulation of thisembodiment can be preferably utilized as, for example, a thermalinsulation being used for a general industrial furnace and requiringprocessing or a thermal insulation for a reformer of a fuel cell.

Next, specific examples according to this embodiment are described.

Example 1 Production of Thermal Insulation

There was produced a dry-pressed compact including anhydrous silica fineparticles (hydrophilic fumed silica fine particles) having an averageprimary particle diameter of about 13 nm and having a thermalconductivity (25° C.) of 0.01 W/(m·K), and a heat-resistant glass fiberhaving an average fiber diameter of 10 μm and having an average fiberlength of 3 mm.

That is, 100 parts by weight of a raw material for a thermal insulationincluding 90 mass % of silica fine particles and 10 mass % of a glassfiber and 0, 1, 3, 5, or 10 parts by weight of calcium hydroxide (ExtraPure Reagent, Wako Pure Chemical Industries, Ltd.) were fed into amixing apparatus, and the mixture was subjected to dry mixing.

Then, the resultant dry-mixed powder was used to produce a dry-pressedcompact having a plate shape of 100 mm×150 mm and 15 mm in thickness bydry-press forming. To be specific, a suitable amount of the dry-mixedpowder was first filled in a forming mold having a predetermineddeaeration system. Then, dry-press forming was carried out so as toprovide a desired bulk density. That is, in the dry-press forming, apressing pressure was adjusted so as to provide the dry-pressed compactwith a bulk density of 250 kg/m³. After the forming, the resultantdry-pressed compact was immediately taken out of the forming mold.

Next, the dry-pressed compact was kept for 3 to 400 hours in athermo-hygrostat at a temperature of 80° C. and at a relative humidityof 90%, or kept for 6 hours in an autoclave at a temperature of 170° C.,to thereby carry out high humidity curing. Then, the dry-pressed compactafter the curing was dried at 105° C. to yield a thermal insulation.

[Evaluation of Compressive Strength]

The compressive strengths of each the thermal insulation were measuredby using a universal testing apparatus (TENSILON RTC-1150A, OrientecCo., Ltd.). That is, a test piece obtained by cutting the thermalinsulation so that the test piece had a dimension of 30 mm×30 mm×15 mmwas loaded with a weight on the press surface (30 mm×30 mm) in thedirection perpendicular to the press surface, and the weight of the loadat the time of fracture of the test piece was defined as the compressivestrength (MPa).

FIG. 3 shows the production condition and compressive strength of eachthermal insulation by relating them to each other. The compressivestrength of the thermal insulations having undergone curing increasedremarkably compared with the compressive strength (0.25 MPa) of thethermal insulations not having undergone curing.

That is, when curing was carried out at 80° C. at 90 RH % with nocalcium hydroxide added (0 parts by weight), the compressive strengthwas improved as the curing time increased. To be specific, 3 hours ofcuring increased the compressive strength up to 0.40 MPa and 400 hoursof curing resulted in a compressive strength of 1.08 MPa.

In the meantime, when curing was carried out in an autoclave with nocalcium hydroxide added (0 parts by weight, A/C), the compressivestrength of the thermal insulation having undergone 6 hours of curingwas 0.97 MPa. Note that, though not shown in FIG. 3, when curing wascarried out in an autoclave at 120° C. or 200° C. with no calciumhydroxide added, it was also confirmed that 6 hours of curing increasedthe compressive strength to a similar extent.

In the meantime, it was possible to enhance the compressive strength ina shorter time in the case where curing was carried out with calciumhydroxide added compared with the case where curing was carried out withno calcium hydroxide added. Further, it was confirmed that there was atendency that as the addition amount of calcium hydroxide was increased,it was possible to enhance the compressive strength in a shorter time.

That is, when curing was carried out at 80° C. at 90 RH % with 1 part byweight of calcium hydroxide added, 3 hours of curing increased thecompressive strength up to 0.83 MPa and 48 hours of curing resulted in acompressive strength of 1.13 MPa.

When curing was carried out at 80° C. at 90 RH % with 3 parts by weightof calcium hydroxide added, 3 hours of curing increased the compressivestrength up to 0.89 MPa and 6 hours of curing resulted in a compressivestrength of 1.03 MPa.

When curing was carried out at 80° C. at 90 RH % with 5 parts by weightof calcium hydroxide added, 3 hours of curing increased the compressivestrength up to 0.91 MPa and 6 hours of curing resulted in a compressivestrength of 1.08 MPa.

When curing was carried out at 80° C. at 90 RH % with 10 parts by weightof calcium hydroxide added, 3 hours of curing increased the compressivestrength up to 0.93 MPa.

On the other hand, in the case where curing was carried out in anautoclave with calcium hydroxide added (1 to 10 parts by weight, A/C),the compressive strength increased to a lower extent compared with thecase where curing was carried out at 80° C. at 90 RH %.

[Electron Microscope Observation]

FIG. 4 shows one example of each of the results obtained by observing,with a scanning electron microscope (SEM), a thermal insulation producedwithout adding any calcium hydroxide and without curing (FIGS. 4(A) and4(B)), and a thermal insulation produced by adding 3 parts by weight ofcalcium hydroxide and carrying out curing at 80° C. at 90 RH % for 24hours (FIGS. 4(C) and 4(D)).

As shown in FIG. 4(A), unevenness was clearly observed on the surface ofthe thermal insulation produced without adding any calcium hydroxide andwithout curing. In contrast, as shown in FIG. 4(C), the surface of thethermal insulation produced by adding calcium hydroxide and carrying outcuring was relatively flat. This must be because, in the thermalinsulation shown in FIG. 4(A), individual silica fine particles simplyaggregated, and in contrast, in the thermal insulation shown in FIG.4(C), silica components eluted from silica fine particles formed abridge structure between the silica fine particles, resulting in thedensification of the internal structure of the thermal insulation.

Further, as shown in FIG. 4(B), on the surface of the thermal insulationproduced without adding any calcium hydroxide and without curing, ablurred boundary was observed between silica fine particles. Incontrast, as shown in FIG. 4(D), on the surface of the thermalinsulation produced by adding calcium hydroxide and carrying out curing,the boundary between silica fine particles was clearly observed. Thiswas probably because, in the thermal insulation shown in FIG. 4(D),silica components eluted from silica fine particles formed a bridgestructure between the silica fine particles, resulting in enhancement ofelectrical conductivity, and hence electron beams were able to bedetected with high sensitivity.

[X-Ray Diffraction]

Each of thermal insulations produced by adding 3, 5, or 10 parts byweight of calcium hydroxide and carrying out curing at 80° C. at 90 RH %for 0 to 24 hours or carrying out curing for 6 hours in an autoclave wassubjected to X-ray diffraction (XRD), analyzing how the content ofcalcium hydroxide and the formation amount of calcium silicate changeddepending on the increase of curing time.

FIGS. 5A to 5D show examples of the XRD measurement results of thermalinsulations produced by adding 3 parts by weight of calcium hydroxide.FIGS. 5A and 5B illustrate the measurement results of a thermalinsulation produced without curing, and FIGS. 5C and 5D illustrate themeasurement results of a thermal insulation produced by carrying outcuring for 24 hours. As illustrated in FIGS. 5A to 5D, peaks of calciumhydroxide (Ca(OH)₂) disappeared and a peak of calcium silicate (CSH)newly appeared because of the curing.

FIGS. 6A to 6C show the curing time, XRD peak values of calciumhydroxide (Ca(OH)₂) and calcium silicate (CSH), and compressive strengthof each thermal insulation by relating them to each other. FIG. 6A showsthe results on thermal insulations produced by adding 3 parts by weightof calcium hydroxide. FIG. 6B shows the results on thermal insulationsproduced by adding 5 parts by weight of calcium hydroxide. FIG. 6C showsthe results on thermal insulations produced by adding 10 parts by weightof calcium hydroxide.

As shown in FIGS. 6A to 6C, it was found that there was a tendency forthe content of calcium hydroxide to decrease with increasing curingtime, resulting in the increase of the compressive strength. That is, itwas found that there was a tendency that as the consumption amount ofcalcium hydroxide increased, the compressive strength of a thermalinsulation increased.

Further, with increasing curing time, new generation of calcium silicateoccurred, leading to the increase of the content of calcium silicate. Itshould be noted that after complete consumption of calcium hydroxide, itwas found that there was a tendency for the compressive strength todecrease though the content of calcium silicate increased.

That is, there was not always a correlation between the content ofcalcium silicate and the increase of the compressive strength of athermal insulation. Rather, it was found that there was a tendency forthe compressive strength to decrease as the content of calcium silicateincreased.

Example 2 Production of Thermal Insulation

There was produced a dry-pressed compact further including siliconcarbide having an average particle diameter of 3 μm in addition to thesilica fine particles and the glass fiber used in Example 1 describedabove.

That is, 100 parts by weight of a raw material for a thermal insulationincluding 75 mass % of silica fine particles, 5 mass % of a glass fiber,and 20 mass % of silicon carbide and 0, 3, 5, or 10 parts by weight ofcalcium hydroxide were fed into a mixing apparatus, and the mixture wassubjected to dry mixing.

The resultant dry-mixed powder was used to produce a dry compact havinga plate shape of 100 mm×150 mm and 15 mm in thickness by dry-pressforming. In the dry-press forming, a pressing pressure was adjusted soas to provide the dry-pressed compact with a bulk density of 240, 260,280 or 300 kg/m³.

Next, the dry-pressed compact including calcium hydroxide was kept for 8hours in a thermo-hygrostat at a temperature of 80° C. and at a relativehumidity of 90%, to thereby carry out high humidity curing. Then, thedry-pressed compact after the curing was dried at 105° C. to yield athermal insulation. In the meantime, a dry-pressed compact free ofcalcium hydroxide was not subjected to curing.

[Evaluation of Compressive Strength and Thermal Conductivity]

The compressive strengths of each the thermal insulation were measuredin the same manner as that in Example 1 described above. Further, thethermal conductivities of each the thermal insulation at 200, 400, or600° C. was measured by a cyclic heat method. That is, a temperaturewave was propagated in a test body, and the phase lag of the temperaturewave was used to determine thermal diffusivity. Then, the thermaldiffusivity and specific heat and density both separately measured wereused to calculate the thermal conductivity. Note that the temperaturewave which was used was a wave of temperature having a temperatureamplitude of about 4° C. and having a period of about 1 hour. Further, atime required for the temperature wave to pass through from one point toanother in the test body was defined as the phase lag. As is well known,the cyclic heat method provides two kinds of ways for thermaldiffusivity measurement; one is a way to measure the phase lag of thegenerated temperature wave which propagates through a specimen, theother is a way to measure the amplitude decay of the temperature wave.Anyhow the physical value obtained from these two ways is the thermaldiffusivity of a specimen, so the thermal conductivity of the specimenis obtained by multiplying the specific heat and the bulk density by thethermal diffusivity.

FIG. 7 shows the addition amount of calcium hydroxide, bulk density,compressive strength, and thermal conductivity by relating them to eachother. Note that the bulk density was calculated from the weight andvolume of a test body. That is, the actual dimension of the test bodywas used to calculate the volume of the test body, and a value obtainedby dividing the weight of the test body by the volume was defined as thebulk density of the test body.

As shown in FIG. 7, it was demonstrated that curing with calciumhydroxide added increased the compressive strength of a thermalinsulation. Further, when the bulk density was set to a given value, thecompressive strength was highest in the case where the addition amountof calcium hydroxide was 3 parts by weight. Further, a significantchange in the thermal conductivity depending on whether or not curingwas carried out was not found.

Example 3 Production of Thermal Insulation

There was produced a dry-pressed compact including silica fineparticles, a glass fiber, and silicon carbide in the same manner as thatin Example 2 described above. That is, 100 parts by weight of a rawmaterial for a thermal insulation including 75 mass of silica fineparticles, 5 mass % of a glass fiber, and 20 mass % of silicon carbideand 0 or 3 parts by weight of calcium hydroxide were fed into a mixingapparatus, and the mixture was subjected to dry mixing.

The resultant dry-mixed powder was used to produce a dry-pressed compacthaving a plate shape of 100 mm×150 mm and 15 mm in thickness bydry-press forming. In the dry-press forming, a pressing pressure wasadjusted so as to provide the dry-pressed compact with a bulk density of240, 260, 280 or 300 kg/m³.

Next, the dry-pressed compact including calcium hydroxide was kept for 0to 24 hours in a thermo-hygrostat at a temperature of 80° C. and at arelative humidity of 90%, to thereby carry out high humidity curing.Then, the dry-pressed compact after the curing was dried at 105° C. toyield a thermal insulation. In the meantime, a dry-pressed compact freeof calcium hydroxide was not subjected to curing.

[Evaluation of Compressive Strength]

The compressive strength of each thermal insulation was measured in thesame manner as that in Example 1 described above. FIG. 8 shows theaddition amount of calcium hydroxide, curing time, bulk density, andcompressive strength by relating them to each other.

As shown in FIG. 8, the compressive strength of thermal insulationsproduced by carrying out curing (3 parts by weight of calcium hydroxidewere added and the curing time was 1 to 24 hours) remarkably increasedcompared with that in the case where curing was not carried out (calciumhydroxide was not added or 3 parts by weight of calcium hydroxide wereadded, and the curing time was 0 hours).

Further, when 3 parts by weight of calcium hydroxide were added, it wasfound that there was a tendency for the compressive strength to increasewith increasing curing time. Note that in the case where 3 parts byweight of calcium hydroxide were added and no curing was carried out,the compressive strength decreased more than that in the case where nocalcium hydroxide was added and no curing was carried out.

Example 4 Production of Thermal Insulation

There was produced a dry-pressed compact including silica fineparticles, a glass fiber, and silicon carbide in the same manner as thatin Example 2 described above. That is, 100 parts by weight of a rawmaterial for a thermal insulation including 75 mass % of silica fineparticles, 5 mass % of a glass fiber, and 20 mass % of silicon carbideand 3 parts by weight of calcium hydroxide were fed into a mixingapparatus, and the mixture was subjected to dry mixing.

The resultant dry-mixed powder was used to produce a dry-pressed compacthaving a plate shape of 100 mm×150 mm and 15 mm in thickness bydry-press forming. In the dry-press forming, a pressing pressure wasadjusted so as to provide the dry-pressed compact with a bulk density of240, 260, or 280 kg/m³.

Next, the dry-pressed compact was kept for 24 hours in athermo-hygrostat at a temperature of 40, 60, or 80° C. and at a relativehumidity of 90%, to thereby carry out high humidity curing. Then, thedry-pressed compact after the curing was dried at 105° C. to yield athermal insulation. In the meantime, a thermal insulation producedwithout adding any calcium hydroxide and without curing was prepared aswell.

[Evaluation of Compressive Strength]

The compressive strength of each thermal insulation was measured in thesame manner as that in Example 1 described above. FIG. 9 illustratescuring temperature, bulk density, and compressive strength by relatingthem to each other. As illustrated in FIG. 9, curing at a temperature of40° C. or more remarkably increased the compressive strength. Further,as the curing time was higher, the compressive strength remarkablyincreased.

[Electron Microscope Observation]

FIGS. 10A and 10B show examples of each of the results obtained byobserving, with a scanning electron microscope, a thermal insulationproduced by carrying out curing without adding any calcium hydroxide(FIG. 10A), and a thermal insulation produced by adding 3 parts byweight of calcium hydroxide and carrying out curing at 40° C. at 90 RH %for 24 hours (FIG. 10B).

As shown in FIGS. 10A and 10B, the surface of the thermal insulationproduced by adding calcium hydroxide and carrying out curing (FIG. 10B)was more smooth compared with that of the thermal insulation produced bycarrying out curing without adding any calcium hydroxide (FIG. 10A).This must be because, in the thermal insulation shown in FIG. 10B, theaddition of calcium hydroxide promoted the elution of silica componentsfrom silica fine particles, resulting in increased densification of theinternal structure.

Example 5 Production of Thermal Insulation

There was produced a dry-pressed compact including silica fineparticles, a glass fiber, and silicon carbide in the same manner as thatin Example 2 described above. That is, 100 parts by weight of a rawmaterial for a thermal insulation including 75 mass % of silica fineparticles, 5 mass % of a glass fiber, and 20 mass % of silicon carbideand 0 or 3 parts by weight of calcium hydroxide or magnesium hydroxidewere fed into a mixing apparatus, and the mixture was subjected to drymixing.

The resultant dry-mixed powder was used to produce a dry-pressed compacthaving a plate shape of 100 mm×150 mm and 15 mm in thickness bydry-press forming. In the dry-press forming, a pressing pressure wasadjusted so as to provide the dry-pressed compact with a bulk density of240, 260 or 280 kg/m³.

Next, the dry-pressed compact was kept for 24 hours in athermo-hygrostat at a temperature of 80° C. and at a relative humidityof 90%, to thereby carry out high humidity curing. Then, the dry-pressedcompact after the curing was dried at 105° C. to yield a thermalinsulation. In the meantime, a thermal insulation produced withoutadding any calcium hydroxide and without curing was prepared as well.

[Evaluation of Compressive Strength]

The compressive strength of each thermal insulation was measured in thesame manner as that in Example 1 described above. FIG. 11 illustratesthe kind of hydroxide added, bulk density, and compressive strength byrelating them to each other.

As shown in FIG. 11, curing with no hydroxide added remarkably increasedthe compressive strength, and curing with calcium hydroxide addedincreased the compressive strength more remarkably. Further, curing withmagnesium hydroxide added also increased the compressive strengthcompared with the case where no hydroxide was added and curing was notcarried out.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications as fall within the true spirit andscope of the invention.

1. A method of producing a thermal insulation, comprising curing adry-pressed compact containing silica fine particles each having anaverage particle diameter of 50 nm or less and a reinforcement fiber, ata relative humidity of 70% or more.
 2. The method of producing a thermalinsulation according to claim 1, wherein the dry-pressed compact is freeof a binder.
 3. The method of producing a thermal insulation accordingto claim 1, wherein the dry-pressed compact comprises 50 to 98 mass % ofthe silica fine particles and 2 to 20 mass % of the reinforcement fiber.4. The method of producing a thermal insulation according to claim 1,wherein the dry-pressed compact comprises at least one of analkaline-earth metal hydroxide and an alkali metal hydroxide.
 5. Themethod of producing a thermal insulation according to claim 4, whereinthe dry-pressed compact comprises at least one of the alkaline-earthmetal hydroxide and the alkali metal hydroxide, each at 0.1 to 10 partsby weight with respect to 100 parts by weight of a raw material for thethermal insulation comprising the silica fine particles, and thereinforcement fiber.
 6. A thermal insulation, which is produced by themethod according to claim
 1. 7. A thermal insulation, comprising silicafine particles each having an average particle diameter of 50 nm or lessand a reinforcement fiber, wherein the thermal insulation has a bulkdensity of 190 to 600 kg/m³ and has a compressive strength of 0.65 MPaor more.
 8. The thermal insulation according to claim 7, wherein thethermal insulation is free of a binder.
 9. The thermal insulationaccording to claim 7, comprising 50 to 98 mass % of the silica fineparticles and 2 to 20 mass % of the reinforcement fiber.
 10. The thermalinsulation according to claim 7, further comprising at least one of analkaline-earth metal and an alkali metal in addition to the silica fineparticles and the reinforcement fiber.
 11. The thermal insulationaccording to claim 10, comprising at least one of the alkaline-earthmetal and the alkali metal, each at 0.1 to 10 parts by weight withrespect to 100 parts by weight of a raw material for a thermalinsulation containing the silica fine particles, and the reinforcementfiber.
 12. The thermal insulation according to claim 7, wherein thethermal insulation has a thermal conductivity at 600° C. of 0.05 W/(m·K)or less.